INCREASE OF ALKALINE-PHOSPHATASE IN MULTIDRUG-RESISTANT TUMOR-CELLS AND THEIR CROSS-RESISTANCE TO 6-THIOGUANINE

The expression of alkaline phsophatase (AP) was analyzed in multidrug- resistant (MDR) tumor cells (sarcoma 180, lung carcinoma, and renal ce ll carcinoma cell lines) by means of immunocytochemistry. MDR cell cul tures showed an overexpression of AP and a cross-resistance to 6-thiog uanine (6-TG, CAS 154-42-7). Significant correlations between AP expre ssion and doxorubicin or vincristine resistance and P-glycoprotein (P- 170) expression were found in these cell cultures. A specific AP inhib itor, levamisole, reversed resistance to 6-TG, but not to doxorubicin. This indicates that 6-TG resistance is certainly associated to P-170 but a causal function of AP for the development of MDR does not exist.